This invention relates to a heat transfer surface on tubular or plate-like bodies having a microstructure of projections protruding out of the base surface, the microstructure being galvanized onto the base surface with a minimum height of 10 xcexcm, as well as a method of producing such heat transfer surfaces.
According to the state of the art, heat transfer surfaces are used in a variety of shapes and sizes in evaporators and condensers. Their structural design will depend on the type of evaporation (convective evaporation, nucleate boiling or film evaporation) and condensation (dropwise or film condensation).
The area of nucleate boiling is of the greatest importance. The formation of vapor bubbles takes place on the heat transfer surfaces. The growth, size and number of bubbles per unit of heat transfer surface and time are determined by essentially three parameters:
a) the properties of the boiling liquid,
b) the material of the heating wall as well as the structure of the heating surface,
c) the heat flow density.
In order for vapor bubbles to be able to develop and grow in a liquid, certain physical conditions must be met. The model concepts for describing these conditions are usually based on homogeneous nucleation, which in turn is usually attributed to fluctuations in density. Once it has formed, a vapor bubble requires an environment that allows it to grow. A simple equilibrium analysis yields the following relationship in evaporation:                               T          -          T∞                =                              2            ⁢            σ            ⁢                          xe2x80x83                        ⁢            T∞                                Δ            ⁢                          xe2x80x83                        ⁢            hςvr                                              (equation I)            
where:
r=bubble radius,
"sgr"=surface tension of the liquid,
xcex94h=enthalpy of evaporation,
xcex4v=vapor density,
T=temperature of the liquid,
T∞=the equilibrium temperature at a planar phase boundary.
The temperature difference Txe2x88x92T∞ may thus be interpreted as the minimum required overheating of the boiling liquid at the prevailing bubble size having radius r. It may be reduced by the fact that bubbles of large dimensionsxe2x80x94i.e., with a large rxe2x80x94are produced through suitable measures. The heating heat transfer surface plays a central role. A favorable design of this heat transfer surface can greatly increase the efficiency of heat transport in boiling. The goal here is to achieve a heat transfer surface having a microstructure, which leads to the highest possible bubble density with a large bubble radius at the smallest possible temperature difference. This is a prerequisite for efficient heat transfer from the heat transfer surface to the liquid.
Essentially microstructures having cavities which are not flooded by the surrounding liquid after the bubbles break away are essentially suitable for this purpose. Vapor bubbles formed in the cavities expand during the growth phase into the liquid adjacent to the heat transfer surface and break way from this heat transfer surface when a system-dependent critical variable is exceeded; this takes place in such a manner that vapor residues remain in the cavities and serve as nucleation seeds for subsequent bubbles.
In the area of condensation, we encounter essentially film condensation and heat transfer devices, where the primary purpose is to keep the thicker condensate film away from the cooling heat transfer surface, which should also be provided with suitable microstructures. The driving force for the runoff of condensate can be linked to the capillary pressure                               Δ          ⁢                      xe2x80x83                    ⁢          p                =                              2            ⁢            σ                    r                                    (equation II)            
where "sgr" is the surface tension and r is the radius of curvature of the phase boundary.
U.S. Pat. Nos. 4,288,897, 4,129,181 and 4,246,057 have disclosed microstructures as heat transfer surfaces on tubular bodies, where smooth tubes are wrapped with layers of polyurethane foam with a thickness of approximately 0.00025xe2x80x3 to 0.0025xe2x80x3 (approximately 6.35 xcexcm to 63.5 xcexcm), their open pore structures first being metal plated in a chemical process. Then the tube is connected to the metal-plated polyurethane sheathing as the cathode and to the base surface of the tube as the anode, and the galvanic deposition is begun. The electrolyte penetrates through the foam to the cylindrical surface of the tube, permitting a uniform deposition of metal ions on the tube and also in the interior of the foam structure. After achieving a suitable layer thickness, the galvanic process is terminated and the foam material is removed by burn-off (pyrolysis, leaving a porous metallic structure that is highly cross-linked and intermeshed on the base surface. It contains completely irregular thicknesses of the webs and completely different cavities and thus completely irregular, unordered structures, leaving the formation of vapor bubbles, e.g., in evaporation, up to chance. In cooling, impurities in the coolant remaining behind in the microfine cavities can have an extremely negative effect on the heat transfer.
U.S. Pat. No. 4,219,078 discloses a heat transfer surface in which a porous film to be wrapped around a tube contains copper particles with a diameter of 0.1 mm to 0.5 mm which are applied to the base surface in multiple layers and are joined by a galvanic process to an entire surface structure. Although this surface structure has a certain regularity, this cannot conceal the fact that bubbling is hindered more than promoted by the multilayered nature of the particles. The numerous cavities also counteract good heat transfer efficiency with regard to film condensation.
To make heat transfer surfaces porous and thus provide them with a certain uniformity in ordered structure with regard to their surface, non-generic mechanical machining processes are often used, such as those disclosed in German Patent 197 57 526 C1, U.S. Pat. No. 4,577,381, German Patent 27 58 526 A1 and European Patent 0 713 072.
Thus, for example, the tubes disclosed in German Patent 197 57 526 C1 and in European Patent 0 057 941 are worked with special rolling and upsetting tools to achieve a special, very rough, knurled surface structure. However, this surface structure is not in the micro range but instead is in the millimeter range, the thickness of the ribs being approximately 0.1 mm and their pitch approximately 0.41 mm with a tube diameter of 35 mm, which does not correspond to the generic microstructure. Although the channel-like cavities beneath the base surface can promote the development of bubbles in evaporation, they counteract the goal of keeping the cooling surfaces free in film condensation. The same thing is also true of the objects of the other publications cited above.
In addition to this previously known state of the art, there are also a number of types of coating by means of a sintering technique, a sprayer technique, flame spraying and sandblasting. None of these are generic methods and none of them attempt to solve the problem on which the present invention is based.
The object of this invention is to create a heat transfer surface of the generic type defined in the preamble as well as a method for producing such a heat transfer surface, which is characterized by an increase in the heat transfer efficiency of its heat transfer surfaces at the lowest possible temperature differences Txe2x88x92T∞ and an optimum thermal efficiency and is suitable for nucleate boiling as well as film condensation at a justifiable manufacturing cost.
This complex object is achieved with regard to the heat transfer surface in combination with the above-mentioned generic term by the fact that the base surface is entirely or partially covered with projections; these projections are applied in the form of ordered microstructures and they have a pin shape, their longitudinal axis running either perpendicular or at an angle between 30xc2x0 and 90xc2x0 to the base surface. This feature creates for the first time a heat transfer surface in the microstructure range whose projections are shaped like pins and extend with their longitudinal axis perpendicular or transversely to the base surface. Therefore, vapor bubbles can lead to unhindered development of bubbles having large dimensions in the microareas between these structures and can develop at the minimum required overheating of the boiling liquid at a temperature difference Txe2x88x92T∞, so that after they break way, new vapor bubbles can form as nuclei and expand in the open cavities, thus ensuring not only a high bubble density but also a high bubble frequency.
Furthermore, the cavities that are completely open to the outside and also between the individual pin-shaped projections may guarantee an excellent film condensation, so that the film can always flow away unhindered and uniformly in all directions. Therefore, an excellent thermal efficiency and an usually high heat transport of heat transfer surfaces designed in this way can be ensured. The heat transfer surface according to this invention also allows variations in the surface density and thickness of the pin-shaped projections, depending on the viscosity of the liquid applied to them, namely between 102/cm2 and 108/cm2 at a thickness between 100 xcexcm and 0.2 xcexcm. The great porosity of this microstructure has a significant positive effect on the heat transfer process in nucleate boiling.
Also in the area of condensation, a heat transfer surface is now created which ensures a good effect of the surface tension "sgr" and promotes heat transport. To achieve a high uniformity of the heat transfer efficiency, it is advantageous to keep a constant length of the pin shape on one and the same heat transfer surface. This length of the pin shape may be between 10 xcexcm and 195 xcexcm, depending on the size and specific function of the heat transfer surface.
It may also be advantageous for the outside configuration of the pin shape on one and the same heat transfer surface to be the same. The thickness of the pin shape may be between 0.2 xcexcm and 100 xcexcm.
Furthermore, it is advantageous to make the clearance between the pin-shaped projections one and the same heat transfer surface regular. This clearance may be between 0.6 xcexcm and 1,000 xcexcm, depending on the desired heat transfer surface and the liquid acting on it.
In the specific embodiment of the pin-shaped projections, this invention also allows numerous embodiments.
For example, according to a first embodiment, the pin-shaped projections are in the shape of a cylindrical column. According to a second embodiment, the pin-shaped projections are designed as cones or truncated cones. According to a third embodiment, the pin-shaped projections may consist of several truncated cones stacked together.
According to a fourth embodiment, the pin-shaped projections are provided with a cylindrical stand whose free end has a mushroom shape.
And finallyxe2x80x94although not exclusively, the pin-shaped projections form a cylindrical stand, whose free end is provided with a spherical shape or a partially spherical shape.
Because of this microstructure, the pin-shaped projections can be applied to practically any plate-like or tubular bodies or similar bodies. However, tubular bodies should have an inside diameter or an outside diameter of at least 2 mm.
The heat transfer surfaces described above are produced according to a method for producing a heat transfer surface on tubular or plate-like bodies with a microstructure which protrudes above the base surface, having a minimum height of 10 xe2x96xa1m of projections galvanized onto it, whereby the base surface is covered with a plastic film and galvanized, as described in U.S. Pat. Nos. 4,288,897, 4,129,181, 4,246,057 and 4,219,078.
In terms of the process technology, the object of this invention is achieved in combination with the aforementioned definition of the generic species by the fact that a polymer membrane which is provided with micropores is applied as a plastic film, so that it covers the entire surface of the base surface, and then in the subsequent galvanization process the body carrying the base surface is wired to function as one of the electrodes, and after reaching the desired length and shape of the pin-shaped projections which form the micropores, the galvanization process is interrupted, and then the polymer membrane is removed.
Due to the shape, the thickness of the polymer membrane, the size and distribution of the micropores in this membrane with regard to their surface density as well as the duration of the galvanization process, it is possible to define the pin-shaped projections which are described above and which in their entirety form the ordered microstructure on the base surface of the heat transfer surface, depending on the requirements of the heat transfer process with regard to the specific properties of the liquid (viscosity, thermal conductivity, surface tension) to meet the needs of the given evaporation or condensation process.
In an especially advantageous refinement of this invention, an ion track membrane, also known as a nuclear trace filter, is used as the membrane, where the micropores in the membrane are formed by ion bombardment and by subsequent etching operation using a base such as an alkaline solution of NaOH.
After conclusion of the galvanization process, i.e., after the final formation of the desired shape and length of the pin-shaped projections, the membrane is stripped off, thereby exposing the entire heat transfer surface.